An interferometric fiber optic gyroscope (FOG) 10, as illustrated in FIG. 1, includes an optical signal light source 12 that provides an optical signal to a fiber optic sensing coil 28. A typical optical signal light source includes a laser diode that provides pump light to a gain fiber. The gain fiber includes a dopant that absorbs pump light and then emits an optical signal that is suitable for delivery. Within the FOG 10, a multifunction integrated optic chip (MIOC) 20 is connected to the fiber optic sensing coil 28. The MIOC 20 includes components such as a polarizer 22, a phase modulator 26 and a Y-coupler (splitter/combiner 24) that are used in processing and controlling optical signals input to and from the fiber optic sensing coil 28. The output of the FOG 10 is an intensity that can be used to determine the phase difference between the two counter-propagating waves. A portion of the output is returned to the light source 12 through a splitter 16 and a second portion of the output is provided to a photodetector 14 through the splitter 16 for measuring the phase difference. The rotation rate of the coil about its sensing axis is obtained by dividing this phase difference by a scale factor of the FOG 10, referred to as the Sagnac scale factor.
The scale factor stability of fiber optic gyroscopes (FOGs) is affected by changes in the polarization state of the light in the fiber between the optical source and the MIOC. Changes in stress within the fiber will cause the polarization state of light guided by the fiber to change. This stress may be mechanical or thermal in origin. Any change in polarization state changes the scale factor of the FOG 10 via optical filtering of the light when traversing first the section between the source and the MIOC 20 (made partly of the single mode fiber and partly of polarization maintaining fiber) then traversing the polarizing MIOC 20 itself. This optical filtering can lead to short term scale factor instability and long term degradation of the scale factor repeatability. A depolarizer 18 (e.g., Lyot depolarizer) can be placed in the optical path between the optical source 12 and the MIOC 20 for depolarizing the optical light source signal to prevent changes in its polarization state as it propagates through the FOG 10. Without the depolarizer 18, random environmental perturbations may degrade the scale factor stability via the mechanism of polarization wavelength asymmetries.
However, the existence of large amplitude polarization non-reciprocity (PNR) bias error(s) in FOGs is in large part attributed to cross-coupling and birefringence temperature sensitivities introduced by the depolarizer. The severity and structure of PNR bias error due to various FOG parameters is summarized in Table I below.
TABLE ISummary of PNR Sensitivity ParametersParameterSymbolEffectPolarization Extinctionε or PERPNR Amplitude RatioCoherence Functionγ(L)PNR AmplitudeCross-CouplingαPNR AmplitudeSagnac Scale FactorKSSFPNR Amplitude Birefringence Temperature Sensitivity      d    ⁢    B        d    ⁢    T  PNR Period
FIG. 2 illustrates a graph of relative PNR amplitude as a function of Lyot depolarizer cross coupling in the prior art FOG of FIG. 1. The cross-coupling is in decibels and is the cross coupling between a first component of the Lyot depolarizer and a second component L2 of the Lyot depolarizer that is coupled at about a 45° angle relative to the first component. As illustrated in FIG. 2, the PNR amplitude reduces proportional to the square-root of the cross coupling, √α. As an example, a 3 dB reduction is cross coupling is predicted to result in a 29% reduction in PNR. Extrapolation of the fit confirms that the predicted PNR amplitude is indeed 29% smaller.
In addition, tests confirm that the severity and structure of the PNR bias error is a strong function of Lyot depolarizer length. FIG. 3 is a graph of PNR bias error versus Lyot depolarizer length for a large sample set of prior art production gyros built with various depolarizer lengths. The severity of the PNR bias error is dependent on the length of the Lyot depolarizer due to variation in the coherence function term, γ(L). Unless utilizing an alternate design, the optimization of the aforementioned parameters is highly constrained.
The parameters required to minimize cross coupling and to select a minimum of the coherence function associated with the Lyot depolarizer results in degraded depolarization performance, which is an unacceptable alternative. An additional example is the MIOC. The design and manufacturing efforts required to enhance the polarization extinction ratio (PER) of the MIOC by a meaningful amount is costly due to the technology limitations of modern integrated optics chip manufacturing and testing. In order to enhance the state of the art (SoA), it is necessary to leverage an alternate FOG configuration that is capable of manipulating PNR sensitive parameters while leaving other design considerations intact.